1. Field of the Invention
The invention is concerned with optical inspection apparatus for monitoring a continuously moving rod, for example a cigarette rod, and for sensing critical variations in the reflective properties of the rod's surface, corresponding to a flaw in the surface. In a tobacco rod such a flaw may be produced by an imperfect lap seal of the wrapper or a tear in the paper produced by a particularly hard piece of tobacco stem.
2. Description of the Prior Art
Conventionally, a cigarette inspection apparatus involves means for measuring the pressure drop caused by leaks in the surface of the cigarette assembly when air suction or pressure is applied. Methods have also been proposed, for example in British Patent Specification No. 1,135,183 or in U.S. Pat. No. 3,626,196, for inspecting a rod by illuminating the rod and detecting the reflected light on a number of photo electric cells. However none of these are entirely satisfactory for high speed production.
In our earlier U.S. Pat. No. 3,854,587, there is described an optical inspection device for monitoring a continuously moving rod and comprising a circular head through which the rod passes, a first set of fibre optic conductors the ends of which terminate at an inner peripheral surface of the head and which transmit light from a source to the head to illuminate the rod passing through the head, and a second set of fibre optic conductors the ends of which also terminate at an inner peripheral surface of the head to pick up light reflected from the rod passing through the head and transmit that light to a photo electric element, such as a photo transistor or photo diode, characterised in that the second set of fibre optic conductors are divided into angularly spaced groups round the head and adjacent groups lead to separate photo electric elements, the outputs of which are fed into separate channels connected to comparator means which is responsive to the signal level in the individual channels and which produces a fault signal if the signal level in any one channel is greater than a pre-set allowable level.
In the absence or rod faults, and as a result of AC coupling to the individual channels, the signal level in the individual channels consists of the AC noise produced by inevitable variation in the reflectivity of the rod as it passes through the inspection head. These variations are due, in the case of cigarette rod, to variation in the reflectivity of the paper wrapper, the variations in tobacco colour seen through the paper, and small variation in the distance of the paper wrapper from the ends of the second set of fibre optic conductors, caused by minor undulations in the rods surface. Faults such as holes, tears, open seams, and dirt spots cause a momentary fluctuation in reflected light which appears as a positive or negative pulse superimposed on the normal noise signal. The division of the second set of conductors into angularly spaced groups, leading to respective ones of the separate parallel channels, ensures that only a proportion of the area of rod surface is viewed by any one photo electric element so that any one channel only carries a noise signal corresponding to the acceptable fluctuations in a thin strip of the rod surface. This effectively increases the fault pulse/noise signal ratio in each channel and hence at the comparator means. The signal level, consisting of the noise signal plus any fault pulse, in each channel is compared with a reference or datum level in a comparator circuit corresponding to that channel, or, as described in the earlier specification, in a common comparator circuit to which the individual channels are connected by a maximum signal selector circuit arranged to transmit to the common comparator circuit the most positive and most negative signal levels existing at any time in any one of the channels.
Difficulties arise in maintaining and optimising the sensitivity and discrimination of the apparatus owing to variations in signal noise from the rod, a drift in the response characteristics of a component over a period of time resulting for example from dust deposit in the inspection head, and differential sensitivity of components related to the separate channels. All these factors can affect the amplitude of the noise signal and, as a result, the apparatus can overlook faults, when a fault pulse is not distinguishable from the noise signal, or incorrectly recognise the noise signal as a spurious fault pulse.